Aligned Electrospun ZnO Nanofibers for Simple and Sensitive

Supplementary Material (ESI) for Chemical Communications
This journal is (c) The Royal Society of Chemistry 2009
Aligned Electrospun ZnO Nanofibers for Simple and Sensitive
Ultraviolet Nanosensors
Zhengtao Zhu,a* Lifeng Zhang,a Jane Y. Howe,c Yiliang Liao,a Jordan T. Speidel,a
Steve Smith,b and Hao Fonga*
Department of Chemistry, b Nano Program, South Dakota School of Mines and Technology, Rapid
City, SD 57701, c Materials Science and Technology Division, Oak Ridge National Laboratory, Oak
Ridge, TN 37831
Preparation of electrospun ZnO nanofibers: The solution for electrospinning was
prepared by dissolving 0.9 g zinc acetate (Aldrich) and 2.7 g polyvinylpyrrolidone (PVP,
Mw = 1,300,000, Aldrich) in 9.5 ml dimethyl formamide (DMF) at the room temperature.
During electrospinning, a positive high voltage of 24 kV was applied through a copper
wire to the solution held inside a glass pipette with the capillary tip diameter of ~0.5 mm.
An electrically grounded copper plate was place approximately 25 cm below the tip of
the pipette. This process of electrospinning was very stable, and the electrospinning jet
ran steadily between the pipette and the copper plate. To obtain uni-axially aligned
nanofibers, a U-shaped metal device with the distance between the two legs of ~25 mm
was used; by manually moving the electrically grounded device back and forth for
several times under the electrospinning jet, tens to hundreds of highly aligned nanofibers
were collected between the legs. After those precursor nanofibers were transferred onto a
silicon (Si) substrate with 300 nm thermal oxide (SiO2) layer, and then pyrolyzed at
500 oC for 4 hours in air to remove the organic components, the electrospun ZnO
nanofibers with diameters of ~200 nm were obtained.
Characterization of electrospun ZnO nanofibers: A Zeiss Supra 40VP field-emission
SEM and a Rigaku Ultima Plus XRD at the South Dakota School of Mines and
Technology, as well as a Hitachi HF-3300 TEM/STEM at the Oak Ridge National
Laboratory, were employed to characterize the morphological and structural properties of
both the as-electrospun precursor nanofibers and the resulting final ZnO nanofibers.
Prior to SEM examinations, the specimens were sputter-coated with gold to avoid charge
accumulations. A rotating X-ray generator (40 kW, 40 mA) with CuKα radiation
(λ = 1.54Å) was used during the XRD experiments. The XRD profiles were recorded
from 10 to 70° at the scanning speed of 2° min-1. The TEM specimens were prepared by
dispersing fibers onto lacey carbon films supported on 200-mesh copper grids. TEM
study was sponsored by the U.S. Department of Energy, the Assistant Secretary for
Energy Efficiency & Renewable Energy, Office of FreedomCAR and Vehicle
Technologies, though the High Temperature Materials Laboratory (HTML) at the Oak
Ridge National Laboratory (ORNL).
Fabrication and evaluation of the UV nanosensors made of uni-axially aligned
electrospun ZnO nanofibers: The UV nanosensors were fabricated by placing the
electrospun (zinc acetate/PVP) precursor nanofibers on the Si/SiO2 substrate with two
gold electrodes, followed by pyrolysis at 500 oC for 4 hours in air. The two electrodes
were lithographically defined with the gap of 250 μm and the width of 3.0 mm. The
Supplementary Material (ESI) for Chemical Communications
This journal is (c) The Royal Society of Chemistry 2009
current-voltage characterization of the ZnO nanosensors were performed in the dark or
under UV irradiation using Keithley 2612 SourceMeters. A 365 nm ultraviolet lamp
(UVO B-100) was used as the light source, and the light intensities were modulated with
a neutral density filter (Ocean Optics) and were measured with a photometer.
Figure S1: A representative SEM image showing the as-electrospun (zinc
acetate/PVP) precursor nanofibers
Figure S2: The current-voltage curves of a typical PVP-coated ZnO nanosensor
under different intensity of UV irradiation